1. Field of the Invention
The present invention relates to ceramics having excellent high-frequency characteristics and is useful for forming a wiring board used in various devices such as packages for containing semiconductor elements, and to a method of producing the same. The invention further relates to a starting composition used for the production of the above ceramics and to a wiring board produced from the above ceramics.
2. Description of the Prior Art
The most widely used wiring board employed for the packages for containing semiconductor elements can be represented by the one in which a wiring layer of a high-melting metal such as tungsten or molybdenum is formed on the surface or inside an insulating substrate that is formed of an aluminaceous sintered product.
At the present time of a highly sophisticated information technology, the frequency band for transmitting signals is shifting toward an ever high frequency region. In a high-frequency wiring board for handling high-frequency signals, the electric conductor forming the wiring layer must have a small resistance and the insulating substrate must have a small dielectric loss tangent at the high-frequency regions, in order to transmit high-frequency signals without loss.
However, a high-melting metal such as tungsten (W) or molybdenum (Mo) has a large resistance and, hence, the wiring board provided with a wiring layer of such a high-melting metal propagates signals at a slow speed and, further, makes it difficult to propagate signals of a high-frequency region of not lower than 1 GHz. It, therefore, becomes necessary to form a wiring layer by using a low-resistance metal such as copper, silver or gold instead of using tungsten or molybdenum.
Due to their low melting points, however, the low-resistance metals cannot be co-fired with alumina which is a material forming the insulating substrate. That is, the insulating substrate and the wiring layer cannot be formed at one time by the co-firing. Therefore, there has been developed a wiring board that can be used as an insulating substrate comprising a so-called glass ceramics (a composite material of glass and ceramics) that can be co-fired with a low-resistance metal.
For example, Japanese Unexamined Patent Publication (Kokai) No. 212363/1990 (prior art A) proposes ceramics obtained by molding and firing a mixture of an AlN powder having an average secondary particle diameter (central particle diameter) of from 3.1 to 8.9 xcexcm and a borosilicate glass powder having an average secondary particle diameter (central particle diameter) of from 1.5 to 16 xcexcm. According to this prior art A, the average secondary particle diameter of the AlN powder and the average secondary particle diameter of the glass powder are controlled to lie within predetermined ranges, in order to enhance the density of the molded article and to improve thermal conductivity of the ceramics.
Further, Japanese Unexamined Patent Publication (Kokai) No. 349166/1992 (prior art B) proposes ceramics obtained by molding a mixture of an Al2O3 powder having an average secondary particle diameter of 1 to 7 xcexcm, an SiO2 powder (10 to 20% by weight) and a PbO powder (20 to 30% by weight) followed by firing at 960xc2x0 C. for 15 minutes. The prior art B teaches the ceramics having a high thermal conductivity and a large strength by using an Al2O3 powder having an average secondary particle diameter of 2.5 to 6 xcexcm.
The ceramics disclosed in the prior arts A and B have advantages of being obtained by the co-firing with a low-resistance metal such as copper, silver or gold, but also have the problems as described below.
That is, in the prior art A, the AlN powder reacts with the glass powder due to firing, whereby fine pores are formed in the ceramics resulting in an increase in the dielectric loss tangent (tan xcex4). Further, AlN has such a property that its tan xcex4 varies depending upon the frequency and exhibits a large tan xcex4 depending upon the frequency band.
The ceramics of the prior art B is excellent in regard to its thermal conductivity and mechanical strength, but exhibits a large tan xcex4 at a high-frequency band due to its large Pb content.
As described above, the ceramics disclosed in the prior arts A and B exhibit large tan xcex4 at high-frequency regions and are not suited for use as the insulating substrate for the wiring board used in, for example, the GHz band due to their large tan xcex4 for high-frequency signals.
It is therefore an object of the present invention to provide ceramics that can be obtained by firing in a temperature region (800 to 1050xc2x0 C.) in which a low-resistance conductor such as gold, silver or copper can be co-fired together therewith, and which exhibits a low dielectric loss tangent at a high-frequency region, and a method of producing the same.
Another object of the invention is to provide a composition useful as a starting material for producing the above ceramics.
A further object of the invention is to provide a wiring board having an insulating substrate formed of the above ceramics, and exhibiting excellent high-frequency transmission characteristics.
According to the present invention, there is provided a starting composition for the production of ceramics, comprising;
(a) a crystallized glass containing SiO2, Al2O3 and MO (M: an alkaline earth metal element) and having a Pb content in an amount of 0 or not larger than 10% by weight in terms of PbO; and
(b) at least one kind of filler having an average primary particle diameter of not smaller than 3 xcexcm and selected from the group consisting of Al2O3, SiO2, MgTiO3, (Mg, Zn)TiO3, TiO2, SrTiO3, MgAl2O4, ZnAl2O4, cordierite, mullite, enstatite, willemite, CaAl2Si2O8, SrAl2Si2O8, (Sr, Ca)Al2Si2O8 and forsterite;
the cyrstallized glass (a) and the filler (b) being contained at a weight ratio a/b of from 30/70 to 99/1.
According to the present invention, further, there is provided ceramics comprising filler crystal particles and a matrix crystal phase formed by a set of a number of crystal particles present on the grain boundaries of the filler crystal particles;
the filler crystal particles comprising at least one kind of oxide crystals selected from the group consisting of Al2O3, SiO2, MgTiO3, (Mg, Zn)TiO3, TiO2, SrTiO3, MgAl2O4, ZnAl2O4, cordierite, mullite, enstatite, willemite, CaAl2Si2O8, SrAl2Si2O8, (Sr, Ca)Al2Si2O8 and forsterite;
the ceramics exhibiting a dielectric loss at 60 to 77 GHz of not higher than 50xc3x9710xe2x88x924.
According to the present invention, further, there is provided a method of producing ceramics, comprising:
a step of preparing the staring composition for the production of ceramics;
a step of molding the starting composition for the production of ceramics into a predetermined shape; and
a step of firing the obtained molded article at 800 to 1050xc2x0 C. for not less than 30 minutes.
According to the present invention, there is further provided a wiring board comprising an insulating substrate and a metallized wiring layer arranged on the surface and/or inside the insulating substrate, the insulating substrate being formed of the above ceramics.
If briefly described, a distinct feature of the present invention resides in the use, in combination, of (a) a crystallized glass containing SiO2, Al2O3 and an alkaline earth metal oxide (MO) and having a Pb content which is suppressed to be not larger than a predetermined value (not larger than 10% by weight in terms of PbO), and (b) a filler having an average primary particle diameter of not smaller than 3 xcexcm and selected from particular inorganic oxide crystals. That is, the starting composition for the production of ceramics containing (a) the crystallized glass and (b) the filler, makes it possible to obtain the ceramics by firing at 800 to 1050xc2x0 C. When the ceramics is used as an insulating substrate of the wiring board, therefore, the wiring board can be produced at one time by the co-firing with a low-resistance conductor such as gold, silver or copper. Further, the ceramics has a crystal matrix with a small Pb content precipitated from the crystallized glass on the grain boundaries of the filler crystal particles having an average particle diameter of not smaller than 2.5 xcexcm. Due to this structure, the ceramics exhibits a small dielectric loss tangent (tan xcex4) at a high-frequency band, i.e., exhibits a dielectric loss tangent (tan xcex4) at, for example, 60 to 77 GHz of not higher than 50xc3x9710xe2x88x924. Therefore, the above ceramics is very useful as a material for constituting the insulating substrate of the wiring board used for the transmission of high-frequency signals.